Compact fast battery charger

ABSTRACT

A compact, economical and fast battery charger for Ni-CAD and Ni-MH batteries turns a current source into a controlled voltage source which keys off a signal representative of the voltage level of the battery during the charging process and turns the current source ON and OFF to assure that the battery during the charging process does not exceed a given voltage level. The apparatus includes in par a ballasting resistor in series with the battery to compensate for the low resistance of the battery and an open collector voltage comparator which establishes high and low voltage set points for turning the current source ON and OFF at the appropriate times. The method and apparatus includes a visual display responsive to a signal which produces on the visual display an indication of the state of charge of the battery during a charging cycle. A circuit board has a long U-shaped trace extending around a notch in the board, on each side of each is mounted on battery. The printed circuit board has a trimming resistor arrangement which allows for trimming to be done using a drill.

This application claims priority of U.S. Provisional Patent ApplicationSer. NO. 60/105,402 filed Oct. 23, 1998.

FIELD OF THE INVENTION

The present invention relates to charging batteries in a fast andefficient manner. More particularly, it relates to charging sealednickel-cadmium (Ni-CAD) and nickel-metal hydride (Ni-MH) batteries witha method and apparatus which operate quickly, efficiently andeconomically.

BACKGROUND OF THE INVENTION

A number of reasonably effective methods and related apparatuses existfor charging sealed Ni-CAD and Ni-MH batteries. Fast charging of Ni-CADand Ni-MH batteries is usually performed with a multiple level currentsource that is controlled by sophisticated mechanisms. These mechanismswork on various schemes including keying off variations in thetemperature as well as methods which employ ratios which work offchanges in the voltage with respect to time, such as the ratio −ΔV/Δt orchanges in the temperature such as ΔT/Δt schemes. The mechanisms aregenerally implemented through integrated circuits. The cost of theseintegrated circuits are prohibitive for many applications in consumerproducts. In addition, most of the available integrated circuits requirean external 3 to 5-volt supply. For autonomous dual cell systems,battery voltage can be as low as 2 volts; therefore, a control circuitwhich overcomes this problem would have to be included.

However, all of these methods and the current apparatuses available havesubstantial expenses related to their implementation in relation to manyof the potential applications for which they can be used. Additionally,many of the devices and circuitry necessary to implement these complexcharging schemes lack the compactness needed for their prospectiveapplications.

Thus, the problem of developing a useable method and apparatus whichwill recharge Ni-CAD and Ni-MH batteries in a relatively quick,efficient and economical manner through use of a compact apparatus hasexisted for quite some time. Ni-CAD and Ni-MH batteries are generallyreadily available and have a wide variety of applications. Manyapplications for their use involve relatively small hand-held electronicdevices in which size and cost represent crucial factors in theirdesign. Such devices include hand-held electric razors and similarconsumer devices.

Voltage termination of fast-charge current is a potentially lessexpensive alternative, but charge termination based on voltage sensingis not recommended by battery manufacturers because of the very lowresistance of these cells. Charging a battery with a voltage source settoo high would result in large currents that would overcharge, damageand perhaps destroy the battery. In addition, Ni-MH and Ni-CAD cellshave negative voltage temperature coefficients. In overcharge, thebattery heats up and its voltage decreases, making the problem even lessmanageable. Lead-acid car batteries, vented Ni-CAD and similar batteriesused in industrial applications, because of their differingcharacteristics, are quite amenable to voltage sensing. However, thesetypes of batteries are ill-suited for use in small consumer products.

One attempt to solve many of these problems is illustrated in FIG. 1.Power bipolar transistor (1) in combination with windings (2) and (3)and Schottky diode (4) is used to transfer energy from the input voltagesource (5) to the two battery cells (6) and (7). Current sense resistor(8) in combination with biasing-circuitry/DC-offset circuitry (9) andbase-drive transistor (10) ensure that the rate of transfer of energy islimited. Base-drive-supply circuit (11) completes the fast rate charger.

The fast rate charge is interrupted through the use of transistor (12)and the voltage dividing elements consisting of trimming resistors (13)dividing resistors (14), diodes (15), (16), base-drive transistor (10)and Schottky diode (4). When biased sufficiently transistor (12) keepstransistor (10) ON thus preventing transfer of energy through transistor(1). The operation of the circuit results in the waveform of FIG. 2.

The above circuit has two very serious deficiencies. First, the batteryvoltage at which fast charge interruption will occur can varyconsiderably from one manufactured unit to the next. This is due to theparameter variances of the the multitude of components in the dividingcircuits. It also is caused by the effect of thebiasing-circuitry/DC-offset circuitry (9) and of the base-drive-supplycircuit (11) and the base current required to turn off transistor (1).Second, the circuit resistance between he energy delivering circuit andbattery cells (6) and (7) is very low. Since the effect of transistor(12) is to effectively regulate the battery voltage, voltage adjustmentmust be very accurate in order to avoid overcharging of the battery.

In an attempt to solve these problems, the circuit of FIG. 1 uses anintricate network of trimming resistors (13). However, this schemerequires that 100% of the unit will have to be trimmed. Thus, thissolution creates its own significant problems. Accurately determiningwhich resistors to trim is very difficult and may even require a twostep trimming procedure. In addition, trimming is perform by manuallycutting the tracks, a very undesirable feature for a product which mustbe mass produced.

The circuit of FIG. 1 also has another undesirable feature, whenever thebattery is fully or nearly fully charged, the power supply completelystops for long period of time as shown in FIG. 2. A visual indicator,not shown on FIG. 1, is then turned off. Although the unit is stillconnected to the voltage source (5), there is no visual indication thatthe unit is working. It would be desirable to always have an indicationthat the power supply is functional and also, if possible, that thevisual indicator should indicate in some manner the state of charge ofthe battery when charging.

Thus, a need exists for a fast, efficient and economical way to rechargesealed Ni-CAD and Ni-MH batteries. In particular, one that can adapt theuse of a voltage termination and sensing technique and do so in aneconomical and efficient manner. A method and apparatus that will chargewith a voltage sensing method and apparatus Ni-CAD and Ni-MH batteries,while overcoming the three major problems associated with thistechnique, namely: low resistance between the batteries, negativetemperature coefficients of the batteries and voltage source setting.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method andapparatus to quickly and efficiently charge sealed Ni-Cad and Ni-MHbatteries (as well as other batteries having similar chargingcharacteristics).

It is a further object to provide a method and apparatus for chargingsealed Ni-Cad and Ni-MH batteries which is economical and easy toimplement and manufacture.

It is yet another object of the present invention to provide a compactapparatus for charging a battery cell.

It is yet another object of the present invention to provide a methodand apparatus which can charge sealed Ni-Cad or Ni-MH with voltagesensing and overcome the difficulties associated with this technique.

It is yet another object of the present invention to provide thecapability of indicating the state of charge or discharge of a batteryat any time through use of a visual indicator.

To accomplish these and other objectives, the present invention providesan apparatus for the fast charging of one or more series connectedNi-Cad or Ni-MH batteries using voltage sensing, which apparatus has: Acurrent source controlled by a control signal to turn on and off asupply of current to the one or more battery cells being charged; avoltage controlled circuit for sensing a voltage across the one or morebattery cells being charged and for controlling the current source andwherein the voltage controlled circuit determines when the one or morebattery cells have reached a high voltage set point to control thecurrent source to interrupt said supply of current, and when the voltagecontrolled circuit determines when the one or more batteries have fallento a low voltage set point to control said current source to restoresaid supply of current, and wherein the voltage controlled circuitcycles through a process of charging the one or more batteries until thehigh voltage set point is reached and then suspends charging until thelow voltage set point is reached and then continuously repeating thecycle so that the one or more batteries will reach and maintain asustained voltage equivalent to the low voltage set point. The apparatusof the present invention generally begins a charging cycle with amaximum duty cycle for fast charging, continues with a progressivelysmaller duty cycle for taper charging and ends the charging cycle with aminimum duty cycle for trickle charging. Thus, the scheme of using acurrent source in conjunction with a voltage sensing scheme in effectturns the current source into a voltage source.

In one aspect of the present invention, it provides a visual indicatorassociated with the voltage controlled circuit to provide a variableindication of a rate of charge of the one or more batteries during thecharging process. This degree of charge may be indicated by an LED orother light source by its rate of blinking depending on the stage of thecharging process. The LED is on at all times during the initial stagesof the charging process when the battery is undergoing a fast charge andthe LED thereafter blinks at a progressively slower rate until thetrickle charge phase is reached. In another aspect of the presentinvention, the function of the visual indicator which is controlled by apulsed signal can be adjusted by the addition of circuit components,such as one or more capacitors to vary the pulse width of the signalswhich turn the visual indicator off and on and thus vary its appearanceto one using the device. In an additional aspect of this invention, itincludes a resistor which connects the base of one of the transistors ofthe comparator with a collector of the other transistor of thecomparator so the comparator can sense high and low set points.

In another aspect of the present invention it provides a variableindication which is a variation in a visually perceptible flashingpattern of the LED wherein the LED is fully on in the fast chargingstate, in the taper charging state the LED duty cycle continuouslyshortens and its period lengthens when approaching the trickle chargingstate and in the trickle charging state the duty cycle is at a minimumand the period is longest. In yet another aspect of the invention theLED is first inverted to indicate the state of charge rather than therate of charge.

In another aspect of the present invention, the voltage controlledcircuit uses a two transistor comparator to sense the high and lowvoltage set points and to turn the current source on and off.

In yet another aspect of the present invention, a ballasting resistor isadded in series with the one or more batteries being charged. Thiscompensates for the low resistance of the batteries.

In another aspect of the present invention it provides a printed circuitboard (PCB) for a handheld electronic device designed for ease ofmanufacture and use. The PCB has a U shaped board with a first andsecond leg. There is a mount for a battery on each legs and a conductivetrace on a perimeter of the U-shape of the board interconnecting themount of the first leg to the mount of the second leg for introducing aballasting resistance in series with the batteries. This trace can alsobe fabricated to include a fuse trace.

According to a further aspect of the present invention, the PCB is alsoconstructed with the ability to vary resistance in a circuit on theboard with an automated trimming technique. In this variation, roundconduits or apertures have been made in the board. The apertures arelocated in conducting strips. The conducting strips interconnectresistors into circuits on the PCB. The apertures act as guides for acutting device such as a drill which when inserted into the aperturessevers the connection of the resistor from the circuit, thereby removinga predetermined amount of resistance from the circuit.

In a further aspect of the present invention, it provides a printedcircuit board (PCB) for a handheld electronic device designed for easeof manufacture and use. This PCB has a first section for holding surfacemounted components and a second section for holding through hole mountedcomponents. The first section and second section are configured suchthat the surface mounted components can be soldered with reflowtechniques on the first section and the through hole components can besoldered using wave techniques on the second section.

In another aspect of this invention it provides a current source of theastable type for limiting an output current under overload conditions.The current source includes a power MOSFET, the drain of which isconnected to a first terminal of a primary winding of a transformer; theMOSFET's source is connected to a first connector of a current sensenetwork; and the MOSFET's gate is connected to: a cathode of a Zenerdiode, a first terminal of a trigger resistor, a first connector of anenergy transfer limiting network and a collector of a bipolar npntransistor. In turn, the trigger resistor has a second terminalconnected to a second terminal of the primary winding the transformerand a voltage source. The voltage source has a cathode connected to afirst terminal of a single winding, an emitter of the bipolar npntransistor, a second connector of the current sense network and an anodeof the Zener diode. The single winding has a second terminal whichconnects to a second connector of the energy transfer limiting network,a first terminal of an input voltage compensation resistor. The inputvoltage compensation resistor has a second terminal which connects tothe base of the bipolar npn transistor and to a third connector of thecurrent sense network. The transformer has a secondary winding the firstterminal of which connects to the anode of a Schottky diode.

The energy transfer limiting network includes a diode and first resistorin parallel. The energy transfer limiting network includes a capacitorwith a first terminal which forms the second connector of the energytransfer limiting network and a second terminal which attaches it inseries with the diode and the first resistor by attaching to a firstterminal of each. A second resistor has a first terminal which attachesto a second terminal of the diode. The second resistor has a secondterminal which attaches to a second terminal of the first resistor whichconnection of the two also forms the first connector of the energytransfer limiting network. The energy transfer limiting network limitsthe amount of energy delivered to the gate circuit composed of 21A, 35and 30. In this preferred implementation it also reduces the transfer ofenergy from the primary winding to the secondary winding under overloadconditions.

The current sense network has a first and second resistor and acapacitor, the first and second resistor each have a first terminalwhich attach to each other and form the first connector of the currentsense network; the capacitor has a first terminal which attaches to asecond terminal of the first resistor, which form together the secondconnector of the current sensing network; and the capacitor has a secondterminal which attaches to a second terminal of the second resistorwhich form together the third connector of the current sense network.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood by an examination of thefollowing description, together with the accompanying drawings, inwhich:

FIG. 1 is a schematic of a prior art circuit;

FIG. 2 is a voltage and current timing diagrams of the prior art circuitdepicted in FIG. 1;

FIG. 3 a is a graph of the battery voltage characteristics over time ofa battery during recharging using the method and apparatus of thepresent invention;

FIG. 3 b is a graph of the current characteristics over time synchronouswith FIG. 3 b during the recharging of a battery using the presentinvention;

FIG. 4 is a schematic block diagram which depicts the major functionalcomponents of the present invention;

FIG. 5 is a detailed schematic diagram of the discrete electricalcomponents which make up the power supply and battery portion of thepresent invention;

FIG. 6 is a detailed schematic diagram of the discrete electricalcomponents which make up the voltage control section of the presentinvention;

FIG. 7 is a depiction of the layout of a printed circuit board whichwould use the present invention;

FIG. 8 provides a view of a portion of a PCB with a trimming aperture;

FIG. 8 a depicts a portion of a PCB where a trimming aperture has beendrilled out to sever a connection.

FIG. 9 is a graph of the voltage level to the output current of acurrent source;

FIG. 10 is a graph of charging current to the battery voltage; and

FIG. 11 is another graph of charging current to the battery voltage

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

1. Detailed Overview of the Operation of the Invention:

FIGS. 3 a and 3 b depict the overall method of the present invention.The following example relates to a two cell battery, but the conceptsare readily adaptable to other configurations of battery cells. FIGS. 3a and 3 b depict a time graph of the voltage and current characteristicsof a battery and charging circuit during the charging cycles of abattery using the method and apparatus of the present invention. Whenthe charging of the battery commences, assuming the battery iscompletely or almost completely discharged, an initial fast chargingstate 81 occurs which lasts typically from 5 to 45 minutes. During thisfast charging period, the current as depicted in FIG. 3 b goes ON at 80and remains ON until the voltage in the battery reaches thepredetermined set point of 2.97 volts 83 as depicted in FIG. 3 a. Thus,during the first fast charging phase as depicted in FIGS. 3 a and 3 b,when the current turns ON, the voltage gradually rises from 2.6 volts to2.97 volts, at which point the current turns OFF. The invention asdescribed below provides the necessary circuitry to elicit thisresponse. The current will then turn back ON when the voltage falls to apredetermined level of 2.83 volts and will remain ON until the voltageof the battery reaches the preset maximum voltage of 2.97 volts. Thus,as depicted in FIGS. 3 a and 3 b, this process of charging the battery,which consists of a current pulse charging scheme has an initial fastcharging mode, a subsequent taper charging mode and then a tricklecharging mode which continues indefinitely. As depicted, the currentturns ON at various time periods in pulses upon sensing that the voltageof the battery has dropped below the predetermined minimum point of 2.83and remains ON until the voltage reaches 2.97 at which point it turnsOFF. This process then continues indefinitely. As depicted in FIGS. 3 aand 3 b, the pulses reduce in width and become less frequent with thepassing of time. The process of the present invention thus relies on acombined method of frequency and pulse-width modulation.

FIGS. 3 a and 3 b are only provided to illustrate the concepts describedherein. The vertical and horizontal scales are highly distorted in orderto emphasize the important detail. For example, fast charging time lastsfor several minutes; whereas, the current pulses generated during thetaper charge period following the fast charging are much more numerousthan illustrated and their duration much shorter. Additionally, in thetrickle charging period following the taper charging period the pulsesbecome much further apart in time. As an example, in the preferredembodiment the length of the pulses during the trickle charge phase isless than 100 milli-seconds. In addition, in the preferred embodiment,each current pulse is in fact composed of much shorter current pulses.The “sub-pulse” period is variable and of the order of 50 micro-seconds.The average of the “sub-pulses” is approximately 1.2 Ampere when thecurrent source is on. This average current is somewhat dependent on manyparameters like output voltage, temperature and others. Also, someslopes of the voltage waveform are greatly exaggerated. For instance inFIG. 3 a the voltage between the last two current pulses if drawn toscale would appear flat.

Also, with a detector strictly based on voltage, the period betweenpulse would, in practice, be random. To create pulses which in a fairlypredictable manner become shorter and regularly spaced in time,capacitors are added to the voltage measurement circuit. The outputvoltage is thus integrated before the voltage detector can make adecision. Addition of these capacitors allows the selection of circuitparameters which result in the generation of pulses with the desiredperiod as indicated elsewhere in this text. FIG. 3 a illustrates theeffect of addition of the capacitors where upon careful examination itwill be noted that the turn-OFF of the current source generally occursfor an output voltage greater than the predetermined set point of 2.97Vand turn-ON occurs generally occurs for an output voltage lower than thepredetermined set point 2.83V. However, for simplicity of the discussionherein it is assumed that turn-ON occurs when the output voltage reachesthe predetermined low set point and turn-OFF occurs when the outputvoltage reaches the predetermined high set point. In addition, it mustbe noted that all numerical information given throughout this text isfor the purpose of illustration only. One skilled in the art could buildor design systems with other settings without deviating from the spiritof the invention.

FIG. 4 provides a very simple overall schematic diagram of the basicfunctional divisions of the invention together with some of the keydiscrete elements. Current source 92 connects to the batteries 94.Voltage controlled circuit 91 senses the voltage in the batteries and inthe preferred embodiment, turns the current source ON and OFF during thecharging process as depicted in FIGS. 3 a and 3 b. The voltagecontrolled circuit 91 turns ON the current source when the voltage fallsbelow 2.83 volts in the preferred embodiment and turns the currentsource 92 OFF when the voltage rises to 2.97 volts. Resistor 40 acts asa ballasting or compensating resistor for batteries 94.

In the preferred embodiment, generally two batteries will be charged.Additionally, the invention can include a visual display 93 of FIG. 4which indicates to the user, of the generally hand-held device, thepoint in the recharging sequence that the system is at, such as thebeginning, middle or end. In the preferred embodiment, as will beexplained in more detail below, the visual display is a light emittingdiode (LED) which is ON continuously during the initial phase ofcharging; during the middle phase the LED blinks quickly thenprogressively slower and as the charging sequence approaches the end offully charging the battery, the LED blinking rate stabilizes to a finalrate.

2. Detailed Description of Various Components:

The balance of this description will provide a detailed statement of thekey components of the present invention as they would be implemented inthe preferred embodiment. In reviewing the descriptions of thecomponents, those skilled in the art will appreciate that some of thesefeatures can, on their own, accomplish some, if not all, of the objectsof the present invention. However, those skilled in the art will alsorealize on reviewing the following description that each componentdescribed when used together with the others not only accomplish all theobjects of the present invention but do so as a harmoniously integratedwhole.

A. Creation of a Reliable Voltage Source:

The present invention adopts a constant current source and turns it, at80 of FIGS. 3A and B, into a voltage source. It does so by keying offthe voltage sensed at the output of the current source and thus turnsthe current source ON and OFF during the battery charging process. To bemost effective, the procedure must create a precise voltage source.However, creation of a precise voltage source would be an expensiveproposition. To avoid this expense and still achieve the desired resultsthe invention includes a ballasting resistor 40 of FIGS. 4 and 5. Thus,ballasting resistor 40 allows for the use of a broader range ofimplementation of the control circuit 93.

The subtlety of the invention is best expressed in a more formalmathematical setting. One common representation of a battery is that ofa voltage source in series with a resistor. Both the resistance and thevoltage source are functions of many parameters. For purpose of theanalysis the resistance is considered to be a function of a singlevariable: the state of charge. Thus:

Battery resistance=Rbat(%charge)

The battery voltage is considered to be a function of its state ofcharge and of the average of the charging current consequently:

Battery equivalent source voltage=Vbat(%charge,Icharge).

As illustrated in FIG. 3 the battery is charged at a fast rate until thehigh voltage set point of the control circuit is reached. At that time,the battery has reached a substantial charge level and its resistancefrom that moment on can be approximated by a constant number:Rb=Rbat(100%). Defining, Rs=R40+Rstray where R40 is the resistance of 40and Rstray is any other resistance in series between the current sourceand the battery, the voltage at the output of the current source is:

Vo(ON)=Vbat(%charge,Icharge)+(Rs+Rb)*Is

Vo(OFF)=Vbat(%charge,Icharge)

where Is is the output current of 92.

Defining the Duty Ratio D as the ratio of the ON time to the sum of theON and the OFF time, the average output current is expressed as:

Icharge=D*Is

Thus, the average current delivered to the battery can be found bysolving for D. The following function is defined:

Battery voltage that would yield D=Vb(D)

To solve for the Vb(D), the high and low voltage set points are definedas Vsh and Vsl respectively. Prior to reaching 80 the duty ratio isfull. Modulation of the current source thus starts when:

Vb(D=1)=Vsh−(Rs+Rb)*Is

It is also clear that if Vb(D) (211) was to reach a level equal to thelow voltage set point modulation would stop thus:

Vb(D=0)=Vsl

Solving for intermediate points is slightly more difficult. Theinvention uses a hysterisis comparator with first order low passfiltering. One can solve steady state differential equations to obtainthe desired function. Such a function is illustrated in FIG. 9. TheX-axis is scaled from zero to 1 as a duty ratio axis and is also scaledfrom zero to Is to as an average charging current axis. The taper chargeperiod which starts at 80 with D=1, proceeds from right to left on FIG.9.

Thus as seen at the terminals of the battery equivalent voltage sourceVbat(%charge,Icharge), the voltage control circuit 93 and all seriesresistances thus convert the current source 92 into a second voltagesource with open circuit voltage of:

Vopen=Vsl

The output impedance of this voltage source is non linear, but can berecognized to be a function of the parameter:${Rth} = {\left( {{Rs} + {Rb}} \right) - \frac{{Vsh} - {Vsl}}{Is}}$

Which allows one to rewrite Vb (D=I) as:

Vb(D=I)=Vsl−Rth·Is

To completely describe the charging process final voltage of the batterymust be determined. The equivalent battery voltage for a fully chargedbattery is obtained from the Tafel curves given by battery manufacturersand from the following equation:

Vbat(full,Icharge)=VTafel(Icharge)−Rb*Icharge   I

Using the information that the charging current is equal to Is prior to80 and the information from FIG. 9, the charger current/voltagecharacteristic is plotted on FIG. 10. The fully charged batterycurrent/voltage characteristic obtained from equation I is also plottedon FIG. 10. The charging current will proceed from left to right on thecharger characteristic line and will stabilize at the trickle currentgiven by the intersection of the two characteristic curves. FIG. 10illustrates the essence of the invention; thus, with a single voltagecomparator one can control a current source to: 1.) Deliver fast chargecurrent to a battery, 2.) Terminate fast charge using a voltage sensingmechanism, 3.) Fully charge the battery, 4.) Limit the trickle currentto an acceptable value, and 5.) Maintain the power supply in operationto obtain a visual indication.

Another advantage of the invention is that after reaching the cutoffvoltage Vsl it does not revert immediately to the trickle charge currentbut goes through a taper period, thus reaching full charge of thebattery more quickly.

Actual selection of Vsh, Vsl, Is and the value of 40 are parameters thatone skilled in the art can freely select. Two governing factors in thisselection are the inequality

Vbat(full,Is)>Vsh−(R 40+Rstray+Rb)*Is  II

and the ratio $\begin{matrix}{{Ratio} = \frac{\left( {{R40} + {Rstray} + {Rb}} \right) \star {Is}}{{Vsh} - {Vsl}}} & {III}\end{matrix}$

To guarantee the operation depicted in FIG. 3 this ratio must be largerthan unity otherwise operation similar to that of FIG. 1 will result. Ifone wishes to design the system to maximize the time spent in fastcharge, the ratio must be as close as possible to unity. Rb is not aparameter guaranteed by battery manufacturer and Rstray may varyespecially when the battery interconnection resistance is not wellcontrolled. Setting 40 to a known value will therefore minimize thevariation in the ratio that would result from variations in Rb andRstray. Equation II is the criteria for guaranteed termination of fastcharge. Setting 40 to a known value has a similar stabilizing effect onequation II as on equation III.

On the other hand one may wish to obtain a more conservative design, asthat illustrated in FIG. 11. This type of optimisation ensures that thetrickle current always remains below a certain limit especially inexpectation of variation of the battery voltage at the end of its life.The characteristic illustrated in FIG. 11 is obtained by selecting avalue of ratio larger than unity. Obtaining a high ratio without theaddition of 40 would imply that the difference between Vsh and Vslshould be small. Obtaining a small and predictable difference betweenthe two set points would not be possible with the simple, economical,able to function at low supply voltage and temperature compensatedcomparator described elsewhere in this text. Should the difference beunpredictable so would be the ratio. The pulse periods illustrated inFIG. 3 would also be unpredictable and would affect the performance forboth implementation of the visual display discussed elsewhere in thetext.

One could actually select 40 to be of zero value, but its additionallows the obtaining of predictable and more reliable behavior whenusing the simple, inexpensive and compact implementation of the voltagecomparator and the current source which this invention provides.

In the preferred embodiment of the present invention, ballasting orcompensating resistor 40, in fact, is a long metal strip 40 as depictedin FIG. 7. Resistor 40 is implanted on a circuit board 110. As depictedin FIG. 7, the strip which is resistor 40 runs between the terminals ofthe batteries. As can be easily conceived by those skilled in the art,strip 40 has been given the appropriate thickness and composition toallow it to act approximately as a 100 milli-Ohm resistor as implementedin the preferred embodiment. 100 milli-Ohm resistors are generallyfairly expensive and, in fact, the strip 40 which has been substitutedas a 100 milli-Ohm resistor provides an extremely cheap and easilyfabricated resistor from a manufacturing point of view.

B. Constant Current Source:

In order for the system of the present invention to operate properly, itneeds a reliable and constant current source. Those skilled in the artwill readily appreciate that a number of options exist which can providea constant current source to achieve the desired results of the presentinvention. However, given the particular objectives of the presentinvention, what in part is needed is a rugged current supply, which iseasy to fabricate, easy to turn OFF and ON and can produce a constantcurrent of up to 0.9 to 1.2 amperes if not more. Thus, the preferredembodiment of the present invention builds the current supply around aMOSFET.

Referring to FIG. 5 power MOSFET transistor 21 in combination withwindings 22 and 23 and Schottky diode 24 is used to transfer energy fromthe input voltage source 25 to the two battery cells 26 and 27. Currentsense network 28 in combination with gate-drive transistor 30 ensuresthat the rate of transfer of energy is limited.

The operation of the power supply is of the astable type and isdescribed as follows. When input voltage 25 is applied, resistor 31provides current to the gate 21A of transistor 21. When the thresholdvoltage of gate 21A is reached transistor 21 turns on. This providesvoltage across winding 32 and energy is transferred to gate 21A oftransistor 21 to complete the turn on process. Capacitor 33 limits theamount of energy delivered to the gate circuitry whereas resistor 34limits the rate at which said energy is delivered. Zener diode 35 limitsthe voltage across the gate 21A of transistor 21. While transistor 21 ison, energy is stored in winding 22 and current increases linearly as afunction of time. When the voltage across current sense network 28 issufficient, gate-drive transistor 30 will turn off transistor 21 andenergy will be delivered to battery cells 26 and 27 through winding 23and Schottky diode 24. When all of the energy contained in winding 23 isdepleted, windings 22, 23 and 32 will resonate with the self-capacitanceof the power MOSFET, transistor 21, and of the Schottky diode 24.

This will provide voltage at the gate of 21A of transistor 21 in excessof its voltage threshold and transistor 21 will turn ON again in a muchshorter time than would be required to turn ON by the action of resistor31 alone.

Those skilled in the art will readily recognize an implementation of aself-oscillating flyback converter with constant peak current. Thoseskilled in the art will also recognize that, by proper selection of theturn ratio, the ON time of transistor 21 can be made generally muchshorter than its OFF time for an intended input and output voltagecombination of ranges. Thus, for the given ranges of input and outputvoltage, the output current of the flyback converter can be made to berelatively constant. In addition, resistor 29 is used to reduce the peakcurrent circulating in winding 22 as input voltage is increased. Thisyields nearly constant output current over a wider input voltage range.

The output current for zero output voltage, assuming Shottky diode 24 isan ideal diode, is thus easily calculated to be 0.5 times the peakcurrent delivered by winding 23. The output current of the flybackconverter appears to be inherently limited. Short-circuit current isthus restricted to a value smaller than the maximum theoretical limit.

This upper bound is calculated assuming transistor 21 remains off untilthe energy in winding 23 is completely discharged before transistor 21turns on again. Unfortunately, since the time required to discharge theenergy in winding 23 is inversely proportional to the voltage appliedacross it, the discharged time and overload conditions will be muchlonger than under normal operating conditions. Since start-up resistor31 always provides current to the gate 21A, transistor 21 will againturn ON before the energy of winding 23 is completely discharged. Thiswill result in an accumulation of energy in the power supply that wouldresult in either excessive output current or in failure of transistor21. One instance of low output voltage can occur in the intendedapplication as follows, the power supply and the battery cells areintended to supply current to DC motor 36 through the use of switch 37.Preventing rotation of the rotor by any means would result in such a lowoutput voltage.

To solve the preceding problem, which is essentially an overloadcondition, the invention increases the time required for resistor 31 toturn on transistor 21 again. The invention accomplishes this by addingdiode 38 and resistor 39 to the circuit. Thus, when transistor 21 isOFF, diode 38 prevents capacitor 33 from discharging through resistor34. The discharging current must flow through resistor 39 which is of amuch larger value than resistor 34. This current circulates in zenerdiode 35 and keeps the gate voltage of transistor 21 at a negativevoltage for as long as this current exceeds the current supplied byresistor 31. This effectively extends the time which transistor 21 ismaintained in an OFF state thus limiting the output current under lowoutput voltage conditions.

C. The Voltage Controlled Circuit:

One of the key components of the preferred embodiment of the presentinvention is the voltage controlled circuit. It monitors the voltagelevels of the batteries and compares it to its high and low set pointsand in response controls the current source by turning it ON and OFF atthe appropriate time. This, in conjunction with the ballasting resistor40, in effect turns the constant current source into a voltage asdescribed above.

In the preferred embodiment, voltage regulation is achieved by meansillustrated on FIG. 6. Battery voltage is fed back to transistor 50through resistive divider 51 and high frequency buffer 52. The magnitudeof the battery voltage will thus determine the “on” or “off” state oftransistor 50. In order for the battery voltage level to accomplish thistransition and also be easily predictable, transistor 50 is connected totransistor 53 and resistor 54. The collector current and collectorvoltage of transistor 50 are thus well defined. The base voltage oftransistor 50 for which a transition from ON to OFF or from OFF to ONoccurs can easily be determined as the operating point for transistor 50is well defined. Unlike the circuit of FIG. 1 which has a number ofcomponents with wide tolerances, the invention only has one parameterwith wide tolerance, the base-emitter voltage of transistor 50.Consequently, the use of this regulation circuit in conjunction withresistor 40 results in a product for which most of the manufacturedunits will not require trimming, a significant improvement over theapproach of FIG. 1 where 100% of the manufactured units must be trimmed.

Transistors 50 and 53 form a open collector voltage comparator 101 withpull-up resistor 55. Thus, the structure of FIG. 6 also has thefollowing inherent advantage in that the relation between the high andlow voltage set points are easily programmed by means of resistor 56.The lower set point is set at the voltage at which the average batteryvoltage is expected to rest when the battery is fully charged. High andlow set points are selected as described above.

The output of the comparator 101 is transmitted to the gate oftransistor 21 through buffer 57. The resulting operation is illustratedin FIGS. 3 a and 3 b. When recharging a fully discharged battery, thepower supply will first be continuously on, as the battery voltage plusthe voltage drop across the series resistance is lower than the highvoltage set point. The power supply is actually delivering pulses ofenergy at a high frequency but can be considered to be continuouslydelivering an average current equal to the power supply current ratingand thus acts as the current source 92. The current source is thus ONand the voltage across the battery and resistor 40 will rise until thehigh voltage threshold is reached at which point the current source willthen be turned off. When the voltage falls below the low threshold thecurrent source is turned on again by charging circuit 91. As the batteryis being recharged, the time spent in the OFF mode increases, and thetime spent in the ON mode becomes smaller. The effective currentdelivered to the battery tapers down from a fast charge rate to tricklerate.

D. Visual Display:

The present invention utilizes the ON-OFF behavior of the chargingcircuit as illustrated in FIGS. 3 a and 3 b to generate an indication ofthe status of the charging process. In the preferred embodiment, diode58 resistor 59 and light-emitting-diode 60 are attached to the anode ofdiode 24 of FIG. 5. Whenever the current source is on, LED 60 will emitlight. Referring to FIG. 3 a, the LED will be continuously on at thebeginning of the charging cycle, when the battery is in a dischargedstate. At 83 it will start flashing at a high rate and slow downprogressively during the charging. When it is fully charged the LED willblink on from time to time.

However, the circuitry must be enhanced to produce current pulses whichwill provide a visible indication of the charging status at all timesduring the charging process. Otherwise, the light-emitting diode willbecome progressively less bright and cease to give any apparentindication of the status in the latter half of the charging process.Components have to be added to broaden out the current pulses and makethem visible near the end of the charging cycle. This objective of thepreferred embodiment is achieved by the addition of appropriate valuedcapacitors 41 of FIG. 5 and 62 of FIG. 6. The timing sequence of thecurrent pulses given appropriately valued capacitors 41 and 62 canproduce a visible indication during the entire charging process

It will be appreciated that the rate of flashing of the LED indicatorsource is determined in the preferred embodiment by the cycle period ofthe voltage controlled circuit. In the embodiment described above, thecycle period is sufficiently long such that the cycle period can bedetected by the human eye as a blinking in the LED 60. By increasing thefrequency of the cycle, for example by reducing a capacitance ofcapacitor 41, the blinking rate may become sufficiently high as toappear to the human eye as a constant ON. In this case, the apparentintensity of the LED 60 would be at full intensity when the duty cycleis at its maximum, and the LED 60 would be at a minimum intensity whenthe duty cycle was at its minimum, i.e. it may even appear to be totallyoff. Of course, during the taper region of the charging cycle, the LED60 would appear to be at a medium level of intensity. It will beappreciated that the LED can be powered by an inverter receiving thecontrol signal so as to provide an indication of the degree of charge inthe battery during use to drive a load, as opposed to the rate ofcharging.

Thus the type of signal displayed by the visual indicator can be variedfrom a perceptible flashing light to one which only varies in intensity.Additionally, an inverter could be added to the circuit which would addthe alternative of a signal increasing in intensity or flashing rate asthe battery is charged. Those skilled in the art will also appreciatethat to make the flashing visually perceptible the flash rate must bereduced to less than 20 flashes a second preferably less than 16 asecond. On the other hand to assure that the only change visible is theintensity of the light the flash rate must be at least 24 times a secondor more.

In a further aspect of the method of this invention two or moredifferent light sources can be provided, the apparent intensity of whichcan be varied to form different colors of uniform hue on anappropriately configured visual display, such as light mixing anddiffusing light pipe. The invention can also provide a variable colorindicator comprising a light mixing and diffusing apparatus, such as alight pipe and two different color light sources arranged to directlight at the entrance window of the light pipe, and a circuitcontrolling the intensity of the light sources to produce a desiredcolor on the exit surface of the light pipe in response to a variablecolor control signal. Preferably, the light sources comprises LED's andthe circuit comprises a pulse width modulated (PWM) intensity signal forpowering a first LED and an inverter for inverting the PWM intensitysignal for powering the second LED.

In its preferred implementation the battery charger can also act as avoltage source to DC motor 36. In order to achieve this dual mode,capacitor 41 is taken out of the circuit to allow for rapid changesbetween the OFF and ON state of the current source. Voltage beingpresent at the DC motor terminal, voltage will be fed to transistor 50through resistor 42 and will modify the comparator voltage set point.Thus when a battery is so discharged that it cannot sustain theappropriate voltage across the motor, the current source will.

E. Circuit Fabrication and Layout:

The preferred embodiment is assembled on a printed circuit board (PCB)110 shown on FIG. 7. To facilitate manufacturing and avoid complicatingthe manufacturing process, all through hole components are mounted on alower section 111 of the board as depicted in FIG. 7. All surface mountcomponents are mounted on a top section 112 of the board. The twosections 111 and 112 do not overlap. Consequently, surface mount section112 can be assembled using reflow techniques whereas the through holesection 111 can be soldered using wave technique. Production yields cantherefore be greatly enhanced. As noted, resistor 40 would normally bedifficult to obtain as a surface mount component. The present inventionas noted above solves this problem by fabricating resistor 40 as a trackprinted on the circuit board as illustrated in FIG. 7. The circuitlayout on PCB 110 of the preferred embodiment of the present inventionas depicted in FIG. 7 has additional features which will be explainedbelow.

F. Adjusting for Negative Temperature Coefficients of the Battery:

The present invention also deals with the negative temperaturecoefficient characteristic of batteries. This particular problem caneasily introduce errors and problems into the charging process anddamage the battery when voltage sensing is used to charge the battery.

The present invention in part accounts for this problem by positioningthe key transistor 50, which itself has a negative temperaturecoefficient, adjacent to the battery, see FIG. 7. Transistor 50, whichis the key transistor in the comparator circuitry 101, see FIG. 6, thenheats up along with the battery. Accordingly, the setting of transistor50 changes in step with the batterie's negative temperature coefficientand thus compensate for the errors introduced by the negativetemperature coefficient of the battery.

More specifically, a typical battery generally has a negative voltagecoefficient; thus, when the battery heats up, its voltage will decrease.For a typical voltage regulator, this will result in an increasedcurrent and in turn additional heating and eventually thermal runawaywhich can seriously damage or destroy a battery. A 2 cell battery has acoefficient of approximately −5 mV/° C. A base emitter voltage of a NPNtransistor has a voltage of approximately 0.6V and a temperaturecoefficient of approximately −2 mV/° C. If a base-emitter voltage isused as a voltage reference to regulate the battery at 2.85V, theresulting regulator temperature coefficient will be approximately −9mV/° C. Consequently, if the battery and the transistor are in closethermal contact, the voltage regulator will behave as a temperaturecompensator and will prevent thermal runaway should the voltage setpoint of the regulator be set slightly higher than the battery voltage.In the preferred embodiment of the invention, battery cells are mountedon the top side of the PC board on each of the two legs 114A and 114B ofthe PC board. Transistor 50 is thus strategically mounted underneath oneof the two battery cells as illustrated in FIG. 7.

Making resistor 40 out of a printed circuit board trace also contributesto temperature compensation. PCB traces are made out of copper and havepositive temperature coefficient. Locating resistor 40 in close thermalcontact with the batteries, as illustrated on FIG. 7, causes resistor 40to increase its resistance should the battery cell heat up. For exampleif the current source amplitude is 1.3 Amp and resistor 40 is 0.1 ohm at25° C. then a 20° C. rise in temperature would generate an extra 10 mVdrop thus contribute to temperature compensation.

G. Fine Tuning the Voltage Controlled Circuit:

Trimming generally entails the removal or disconnecting of specificcircuit elements from a circuit to adjust the tolerances of the circuit.Quite often trimming involves the disconnecting of resistance from thecircuit which has been specifically included for this purpose. As notedabove, one of the advantages of the present invention is that itrequires little or no trimming to adjust the circuit during themanufacturing process to provide for appropriate tolerances so that theinvention operates properly. This is in contrast to the circuit of FIG.1 where generally trimming has to be done all the time to adjust forvariations inherent in the circuitry components. The present invention,in its preferred embodiment, limits the adjustment to only once if atall necessary.

More specifically, the base-emitter voltage of a transistor, such as 50,has a statistical distribution which is wide. The resistor networks 51and 61FIG. 6 which are used to scale the battery voltage down to thebase-emitter voltage also have a statistical distribution of possiblevalues. This results in a statistical distribution of regulator setpoints which sometimes exceed that which is acceptable to charge thebattery. Although, the present invention has significantly minimized theneed for trimming to adjust the voltage set points some adjustment maybe necessary from time to time; thus, means must still be availableprovide for such adjustments when the need arises. Voltage adjustmentcan be performed by removal of one or more of the resistors of thevoltage adjustment network 61 (see FIGS. 6 and 7). Manual removal ofsurface mount resistors similar to that depicted in FIG. 1 is timeconsuming and therefore costly. To avoid these problems, the presentinvention not only reduces the number of trim procedures to one at most,it also provides a means to fully automate the trimming procedure whenit is necessary. The present invention achieves this result by providingcutting guides which amount to holes or apertures 63 on the PCB in theconducting lines which connect a potential item to be trimmed to the acircuit on the board. These apertures are positioned to guide anautomated drill bit as it drills into the board in the process ofsevering a component, such as a resistor from a circuit.

The conductive trimming guides depicted schematically as 63 of FIG. 6are shown in FIG. 7 as 163. FIG. 8 provides a magnified view of oneconductive trimming guide consisting of conducting material 207 andaperture 203. The aperture 203 located on PCB 201 lies in conductingscript 202. In fact a portion 207 of conducting strip 202 surrounds theaperture 203. Generally, conducting strip 202 including the portion 207surrounding the aperture 203 are made of a conducting a material such ascopper.

A tip of a drill 209 is depicted poised above aperture 203. The drillbit 209 has a diameter greater than the combining the diameter ofaperture 203 and the surrounding conductive material 207. The drill bit209 is direct towards the center of the aperture 203. Thus, whenrotating drill bit 209 reaches it aperture 203 guides the drill as itcuts into PCB 201. After making of the cut into PCB 201 and beingretracted the drill bit 209 leaves a clean cut hole 211 as depicted inFIG. 8 a. Since the cut by the drill bit 209 leaves a hole 211 with alarger diameter than that of the original aperture 203 and thesurrounding conductive material 207 a clean cut of the conducting strip202 had been made. The complete removal of all of the conductingmaterial 207 surrounding aperture 203 severs conducting strip 202 intotwo unconnected parts 202 a and 202 b. Consequently, any componentsconnected into a circuit by conducting strip 202 are severed from thecircuit.

Since those skilled in the art are filly familiar with the the standardprocedures for fabricating PCB's a detailed discussion herein is notnecessary. Suffice it to say that the typical PCB is manufactured from afiberglass or similar sheet of non-conductive material. A layer ofconductive material is then deposited over one side of the PCB. Thisconductive material generally is copper; however, silver, aluminum orgold are among other possible choices depending upon the application.

Holes to mount of the components are made in the board. Additionally, atthis time the apertures 203 which will form part of the conductingtrimming guides can be made in the board. In fact the same drills usedto form the holes used to secure the through hole components could drillthe holes for the apertures. The copper or other conducting materialdeposited on the board is then etched in the standard fashion. Theetching removes most of the conducting material leaving only anintricate network of conducting strips which will electrically connectthe components to be mounted on the board. Generally, given the verystringent tolerance requirements for manufacturing a PCB, fabrication ofthe apertures is easily included. Thus, the automated trimming can bedone under more imprecise machining conditions during the PCB test phaseafter the addition of the components. Also, the apertures allow for amanual cutting with a drill held by a person which still would retainthe precision necessary to avoid gross mistakes.

While the invention has been particularly shown and described withreference to a preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and detail may bemade to it without departing from the spirit and scope of the invention.

What is claimed is:
 1. An apparatus for fast charging of at least oneseries connected sealed Ni-CAD or Ni-MH battery using voltage sensing,which apparatus can also indicate a state of charge of the at least onebattery during the fast charging, said apparatus comprising: a currentsource controlled by a control signal to turn ON and OFF a supply ofcurrent to said at least one battery; a voltage controlled circuit forsensing a voltage across said at least one battery and for controllingsaid control signal, wherein said voltage controlled circuit determineswhen the at least one battery has reached a high voltage set point tocontrol said current source to interrupt said supply of current, andsaid voltage controlled circuit determines when the at least one batteryhas fallen to a low voltage set point to control said current source torestore said supply of current, and wherein the voltage controlledcircuit performs an iterative cycle of charging the at least one batterywhen the voltage falls to the low voltage set point until the highvoltage set point is reached and continuously repeats the cycle so thatthe at least one battery will reach and maintain a sustained voltageequivalent to the low voltage set point, the voltage controlled circuitbeing constructed to charge the at least one battery with a variableduty cycle beginning a charging cycle with a maximum duty cycle for fastcharging, continuing with a progressively smaller duty cycle for tapercharging and ending said charging cycle with a minimum duty cycle fortrickle charging; and a visual indicator responsive to the controlsignal over time, which indicator provides a variable indication of arate of charge of said at least one battery.
 2. The apparatus of claim 1wherein the visual indicator is LED.
 3. The apparatus of claim 2 whereinthe variable indication is a variation in a visually perceptibleflashing pattern of the LED wherein the LED is fully on in the fastcharging state, in the taper charging state the LED duty cyclecontinuously shortens and its period lengthens when approaching thetrickle charging state and in the trickle charging state the duty cycleis at a minimum and the period is longest.
 4. The apparatus of claim 2wherein said control signal varies so that a flashing rate of the LEDgenerated thereby allows the variable indication of charge of thebattery to be seen as an intensity variation.
 5. The apparatus of claim3 wherein the signal to the LED is first inverted to indicate the stateof charge rather than rate of charge.
 6. The apparatus of claim 4wherein the signal to the LED is first inverted to indicate the state ofcharge rather than rate of charge.
 7. The apparatus of claim 1 whereinthe signal the visual indicator is responsive to comprises pulses ofcurrent.
 8. The apparatus of claim 1 wherein the at least one batterycomprises two cells connected in series.
 9. The apparatus of claim 1wherein the visual indicator responsive to the control signal over timealways provides a human interpretable indication of the state of charge.10. An apparatus for fast charging of at least one series connectedNi-CAD or Ni-MH battery using voltage sensing, said apparatuscomprising: a current source controlled by a control signal to turn ONand OFF a supply of current to said at least one battery; a voltagecontrolled circuit for sensing a voltage across said at least onebattery and for controlling said control signal, wherein said voltagecontrolled circuit determines when the at least one battery has reacheda high voltage set point to control said current source to interruptsaid supply of current, and said voltage controlled circuit determineswhen the at least one battery has fallen to a low voltage set point tocontrol said current source to restore said supply of current, andwherein the apparatus performs an iterative cycle of charging the atleast one battery until the high voltage set point is reached andsuspending charging until the low voltage set point is reached and thenrepeating the cycle a multitude of times so that the at least onebattery will reach and maintain a sustained voltage equivalent to thelow voltage set point, the apparatus being constructed to charge the atleast one battery with a variable duty cycle beginning a charging cyclewith a maximum duty cycle for fast charging, continuing with aprogressively smaller duty cycle for taper charging and ending saidcharging cycle with a minimum duty cycle for trickle charging; whereinthe voltage controlled circuit includes a two transistor comparatorwhich has a resistor which connects the base of a first transistor ofthe comparator with the collector of a second transistor of thecomparator and whereby the two transistor comparator can thus sense thehigh and low set points.
 11. The apparatus of claim 10 wherein the atleast one battery and the comparator are in thermal contact whichfacilitates thermal compensation, and wherein the thermal compensationresults from negative temperature coefficients of the at least onebattery and at least one of the transistors of the comparator andthereby prevents overcharging of the battery.
 12. The apparatus of claim10 wherein the at least one battery comprises at least two cellsconnected in series.
 13. An apparatus for fast charging of at least oneseries connected Ni-CAD or Ni-MH battery using voltage sensing, saidapparatus comprising: a current source controlled by a control signal toturn ON and OFF a supply of current to said at least one battery; aballasting resistor, in series with said at least one battery, tocompensate, during the charging process, for a low resistance of the atleast one battery and/or to compensate for uncertainty on the value ofstray resistance and resistance of the at least one battery; a voltagecontrolled circuit for sensing a voltage across said at least onebattery and for controlling said control signal, wherein said voltagecontrolled circuit determines when the at least one battery has reacheda high voltage set point to control said current source to interruptsaid supply of current, and said voltage controlled circuit determineswhen the at least one battery has fallen to a low voltage set point tocontrol said current source to restore said supply of current andthereby create a voltage source to charge the battery, and wherein theballasting resistor matches the voltage source created with the at leastone battery to charge the at least one battery; and wherein theapparatus performs an iterative cycle of charging the at least onebattery until the high voltage set point is reached, the voltage sensedacross said battery being increased by said resistor, and thensuspending charging until the low voltage set point is reached and thencontinuously repeats the cycle so that the at least one battery willreach and maintain a sustained voltage equivalent to the low voltage setpoint, the apparatus being constructed to charge the at least onebattery with a variable duty cycle beginning a charging cycle with amaximum duty cycle for fast charging, continuing with a progressivelysmaller duty cycle for taper charging and ending said charging cyclewith a minimum duty cycle for trickle charging.
 14. The apparatus ofclaim 13 wherein the ballasting resistor is a conducting strip.
 15. Theapparatus of claim 14 wherein the conducting strip is in thermal contactwith the at least one battery and thereby provides temperaturecompensation.
 16. The apparatus of claim 14 wherein the strip has aresistance of approximately one half to four times the resistance of theat least one battery.
 17. The apparatus of claim 13 wherein the chargingcircuit has a two transistor comparator to sense the voltage levels. 18.The apparatus of claim 17 wherein one of the transistors can adjust fora negative temperature coefficient of the at least one battery andthereby prevent over charging of the at least one battery.
 19. Theapparatus of claim 18 wherein the transistor which adjusts for thenegative temperature coefficient of the batteries has its own negativetemperature coefficient and is placed so that it can sense the heat ofthe at least one battery during charging and thereby adjust for thenegative temperature coefficient of the at least one battery.
 20. Theapparatus of claim 13 further comprising: a visual indicator responsiveto the control signal over time providing a variable indication of adegree of charge of said at least one battery; wherein the voltagecontrolled circuit includes a two transistor comparator which has aresistor which connects the base of a first transistor of the comparatorwith the collector of a second of transistor of the comparator andwhereby the two transistor comparator can thus sense the high and lowset points; and wherein the at least one battery and the comparator arein thermal contact and thereby create thermal compensation.
 21. Theapparatus of claim 13 wherein the at least one series connected Ni-MH orNi-CAD battery comprises two Ni-MH or Ni-CAD cells connected in series.22. The apparatus of claim 1 wherein taper charging is obtained bysetting the ratio of the battery resistance multiplied by the currentsource amplitude to the difference of the comparitor voltage settingwhich results in: charging of a fully discharged battery through thefast, taper and trickle charging, and continuous operation which allowsthe display of the charging status.
 23. The apparatus of claim 22wherein the effective battery resistance can be increased by aballasting resistor.